Vacuum cleaners are required to separate dirt and dust from an airflow. Dirt and dust-laden air is sucked into the appliance via either a floor-engaging cleaner head or a tool connected to the end of a hose and wand assembly. The dirty air passes to some kind of separating apparatus which attempts to separate dirt and dust from the airflow. Many vacuum cleaners suck or blow the dirty air through a porous bag so that the dirt and dust is retained in the bag whilst cleaned air is exhausted to the atmosphere. In other vacuum cleaners, cyclonic or centrifugal separators are used to spin dirt and dust from the airflow (see, for example, EP 0 042 723). Whichever type of separator is employed, there is commonly a risk of a small amount of dust passing through the separator and being carried to the fan and motor unit, which is used to create the flow of air through the vacuum cleaner whilst it is in operation. Also, with the majority of vacuum cleaner fans being driven by a motor with carbon brushes, such as an AC series motor, the motor emits carbon particles which are carried along with the exhaust flow of air.
In view of this, it is common for a filter to be positioned after the motor and before the point at which air is exhausted from the machine. Such a filter is often called a ‘post motor’ filter.
There is an increasing awareness among consumers of the problem of emissions, which can be particularly problematic for asthma sufferers. Thus, recent vacuum cleaner models are fitted with filters which have a large surface area of filter material, and the filters often comprise several types of filter material and a foam pad. Such filters are physically bulky and housing such filters in the cleaner is quite challenging. A vacuum cleaner called the Dyson DC05, manufactured and sold by Dyson Limited, houses a circular post motor filter beneath the dirt collection bin. Air is ducted to a first face of the filter, passes through the filter, leaves the second face of the filter and exhausts from the machine via a set of apertures.
There is also a desire to increase the rate of flow of air through a vacuum cleaner. A higher rate of flow generally increases both the ability of the cleaner to pick up material from a surface and the ability of the cyclonic separator to separate material from the dirty airflow. However, an increased rate of airflow can cause the machine to be noisy in operation. It is possible to place acoustically absorbent material in the path of the exhaust air, but this increases the resistance of the path seen by the airflow. This has a detrimental effect on the overall rate of airflow through the machine in addition to adding both weight and cost to the machine.